Today's semiconductors, whether discrete power or logic ICs, are smaller, run faster, do more and generate more heat. Some microprocessors dissipate power levels that were once the exclusive domain of discrete power devices, namely 10 to 25 watts. These power levels require thermal management techniques involving large capacity heat sinks, good airflow and careful management of thermal interface resistances. A well-designed thermal management program will keep operating temperatures within acceptable limits in order to optimize device performance and reliability.
Semiconductors are kept within their operating temperature limits by transferring junction generated waste heat to the ambient environment, usually the surrounding room air. This is best accomplished by attaching a heat sink to the semiconductor package surface thus increasing the heat transfer between the hot case and the cooling air. Once the correct heat sink has been selected, it must be carefully joined to the semiconductor package to ensure efficient heat transfer through this newly formed thermal interface. Thermal resistance is minimized by making the joint as thin as possible, increasing joint thermal conductivity by eliminating interstitial air and making certain that both surfaces are in intimate contact.
Attaching a heat sink to a semiconductor package requires that two solid surfaces be brought together into intimate contact. Unfortunately, no matter how well prepared, solid surfaces are never really flat or smooth enough to permit intimate contact since all surfaces have a certain roughness due to microscopic hills and valleys. As two such surfaces are brought together, only the hills of the surfaces come into physical contact. The valleys are separated and form air-filled gaps. When two typical electronic component surfaces are brought together, less than one percent of the surfaces make physical contact with as much as 99% of the surfaces separated by a layer of interstitial air. Some heat is conducted through the physical contact points, but much more has to transfer through the air gaps. Since air is a poor conductor of heat, it should be replaced by a more conductive material to increase the joint conductivity and thus improve heat flow across the thermal interface.
Several types of thermally conductive materials can be used to eliminate air gaps from a thermal interface, including greases, reactive compounds, elastomers and pressure sensitive adhesive films. All of these thermal interface materials (TIMs) are designed to conform to surface irregularities, thereby eliminating air voids and improving heat flow through the thermal interface.
Elastomers do not flow freely like the greases or compounds, but will deform if sufficient compressive load is applied to conform to surface irregularities. At low pressures, the elastomer cannot fill the voids between the surfaces and the thermal interface resistance is high. As pressure is increased, more of the microscopic voids are filled by the elastomer and the thermal resistance decreases. For most high durometer materials, mounting pressures around 300 to 500 psi eliminate the interstitial voids and reduce interface resistance to a minimum. Mounting pressure must be permanently maintained by using fasteners or springs to hold the two surfaces together.
Thermally conductive adhesive tapes are double-sided pressure sensitive adhesive films filled with sufficient ceramic powder to balance their thermal and adhesive properties. The adhesive tape is usually supported either with an aluminum foil or a polyimide film for strength and ease of handling. Polyimide support also provides electrical insulation. Adhesive tapes perform much like the elastomeric films, in that they also require some initial mating pressure to conform to irregularities in the mating surfaces. They are also unable to fill large gaps between non-flat surfaces. However, once the joint is formed, the adhesive tapes require no mechanical support to maintain the mechanical or thermal integrity of the interface.
Adhesive tapes provide convenience in attaching a heat sink to a semiconductor package because, unlike liquid adhesives, no cure time is required. The film is applied to one of the surfaces, usually to the heat sink, and it is then forced into contact with the semiconductor package to complete the thermal joint. The application pressure is typically 10 to 50 psi for a few seconds duration. The bond thus formed can be considered permanent and the heat sink is reliably attached to the semiconductor. However, this convenience comes at a price in that tapes are only slightly better at thermal conduction than a dry joint. This is because the thermal tapes do not fill gaps as well as liquids, and thermal joints made with tapes will normally include considerable interstitial air gaps. For the most part, the quality of the two joining surfaces will determine the amount of contact that can be achieved and the thermal performance that can be expected. The high shear strength of these thermal tapes means that reliable joints between heat sinks and semiconductors can be achieved, even with poor surfaces and no mechanical fasteners.
In summary, a variety of materials and approaches are available to manage or minimize the thermal resistance of semiconductor package-to-heat sink interfaces. Thermal greases and compounds will provide the lowest interface resistance, but they are pastes and require care in handling. Elastomers eliminate handling problems but they sometimes require high compressive loads even with well-prepared surfaces. Thermal tapes offer great convenience but their gap filling properties are limited. The success of any particular combination of heat sink, interface material and heat sink will depend on the thoroughness of the design, the quality of the interface material and its proper installation.
A TIM is typically made from a polymer matrix and a highly thermally conductive filler. TIMs find three application areas in a CPU package: 1) to bring a bare die package into contact with heat sink hardware (FIG. 1A), 2) to bring the die into good thermal contact with an integrated heat spreader (FIG. 1B), and 3) to bring the integrated heat spreader (heat spreader) into contact with OEM applied hardware (FIG. 1B). The TIM between the die (or die package) and heat spreader is called a TIM 1 and the TIM between the heat spreader and heat sink hardware is called a TIM 2.
FIG. 2 is an illustration of an arrangement of a non-fusible particle filler material within the polymer matrix of a TIM. The polymer matrix may be a material that can be applied as a paste such as a dispensable syringe or by screen printing. The polymer matrix may also act as an adhesive to bond the two mating parts together. The non-fusible particles, such as most metals, benefit from a high thermal conductivity, however a thermal flow path through the TIM is limited by the point-to-point contact of the particles as shown by the arrows. Non-fusible particles refer to particles that will not melt and flow during packaging assembly process, reliability testing, and product operation and so remain as point contacts with each other. Fusible materials can be defined as materials such as solder-like materials that melt below approximately 300° C. Non-fusible materials will melt well above 300° C., such as aluminum at 660° C., silver at 961° C., copper at 1084° C., gold at 1064° C., etc. Thermal conductivity through these non-fusible particles within the TIM, are limited to percolation and as a result, a thermal bottleneck through the non-fusible particles will be these small or point contacts between the particles.
The phenomenon of percolation describes the effects of interconnections present in a random system, here the number of filler particles that are randomly in point contact with each other to allow thermal conduction. Normally, to improve conduction limited by percolation, the amount of filler could be increased until a threshold amount is reached and heat conduction due to the filler, transitions to a sufficiently high value. Another problem is that for some metal particles in contact with some polymer binders, the bare particle filler can poison the polymer cure such as by hindering or blocking the curing agent.